Conventionally, in the case of a tungsten electrode (hereinafter also referred to as a “tungsten electrode material”, an “electrode material”, or simply an “electrode”) which requires the phenomenon of thermionic emission, thorium oxide is contained in the electrode for use as, for example, the cathode of a discharge lamp or the like with a high heat load for the purpose of improving the thermionic emission properties at a high temperature.
However, thorium is a radioactive element and thus, in terms of safety management, there has been proposed a number of techniques that aim to optimize the selection and composition ratio of a thermionic emission substance adapted to replace thorium oxide.
For example, Patent Document 1 discloses an electron emission material containing W, Ta, Re, or an alloy thereof and, as a thermionic emission substance, a ternary oxide of a Group IIIB metal selected from Sc, Y, and the lanthanides La through Lu and a Group IVB metal selected from Hf, Zr, and Ti, a ternary oxide of a Group IVB metal selected from Hf, Zr, and Ti and a Group IIA metal selected from Be, Mg, Ca, Sr, and Ba, a mixture thereof, or a compound thereof.
It is described that the electron emission material is produced by blending a high-purity tungsten powder or another refractory alloy powder with an additive powder, consolidating the blended powder into a rod form at a high pressure, sintering the rod to a required density at a high temperature, swaging or forging the rod into a rod form with a higher density and smaller diameter, and then machining the rod to the size of an electrode.
Patent Document 2 discloses a short-arc high-pressure discharge lamp in which at least a material of a cathode tip portion contains tungsten and, additionally, as a thermionic emission substance, lanthanum oxide La2O3 and at least one kind of another oxide selected from the group of hafnium oxide HfO2 and zirconium oxide ZrO2.
Further, Patent Document 3 discloses a discharge lamp electrode whose recrystallization temperature is 2000° C. or higher, wherein the cathode or anode comprises one or more kinds of tungsten with a purity of 99.95% or more, doped tungsten in which an alkali metal is added at 100 ppm or less (excluding 0 ppm) to tungsten, and a tungsten-based material in which at least one kind of oxides of cerium, lanthanum, yttrium, strontium, calcium, zirconium, and hafnium is added at 4 wt % or less (excluding 0 wt %) to tungsten. These oxides are cited as thermionic emission substances.
This electrode is produced by applying CIP treatment to a powder in which cerium oxide is added to a tungsten powder, to thereby obtain a compact, processing this compact into a shape close to a final shape of the electrode, then sintering the compact in a hydrogen atmosphere at 1800° C., then performing HIP treatment in an argon gas atmosphere at 2000 atm and 1950° C., and then grinding the obtained sintered body.
Patent Document 4 discloses a high-load and high-intensity discharge lamp, wherein its cathode has a structure in which an oxide of at least one kind of metal selected from lanthanum, cerium, yttrium, scandium, and gadolinium and an oxide of at least one kind of metal selected from titanium, zirconium, hafnium, niobium, and tantalum are coexistent in a high melting point metal base composed mainly of tungsten, and wherein the conversion particle size of the coexisting substance is 15 μm or greater and the plurality of coexisting substances are present in the high melting point metal base.
It is disclosed that the cathode is produced by the following processes. That is, first, a lanthanum-metal oxide powder having an average particle size of 20 μm or less and a zirconium-metal oxide powder having the same average particle size of 20 μm or less are mixed in a ball mill and sintered in the atmosphere at about 1400° C. after pressing. Then, the sintered body is again pulverized to obtain an oxide powder in which the lanthanum-metal oxide and the zirconium-metal oxide are coexistent. Then, the obtained oxide powder is classified to obtain a powder having a particle size of 10 to 20 μm. This powder and a tungsten powder having a purity of 99.5 wt % or more and an average particle size of 2 to 20 μm are mixed together, pressed, presintered in hydrogen, and then normally sintered by applying electric current, thereby producing the cathode.
Herein, conventionally, there are several techniques for measuring the work function which is a value representing the electron emission properties of a material.
Roughly classified, there are known a method of measurement from electron emission by light and a method of measurement from electron emission by heat (hereinafter referred to as thermionic emission).
The method of measurement from electron emission by light is a method of obtaining the work function as average information of the entire emission surface by the phenomenon of photoelectric effect in which electrons are emitted upon irradiation of ultraviolet light or X-ray on the solid surface. This measuring method obtains the work function by the photoelectric effect in the atmosphere at ordinary temperature and thus is intended for a semiconductor or an organic compound which is used around the ordinary temperature (Patent Document 5).
According to Non-Patent Document 1, the photoelectric effect is given by the following equation (Non-Patent Document 1).(mv2)/2=hν−φ
where m is the mass of an electron, v is the maximum speed of the emitted electron, υ is the frequency of irradiated light, h=2πh is the Planck's constant, and φ is the work function. Herein, the photoelectric effect represents the behavior of a particle having an energy of hν.
On the other hand, the method of measurement from thermionic emission is a method of measuring a current by thermionic emission (hereinafter referred to as a thermionic emission current) and deriving the work function of a material from a current value thereof. For example, in Patent Document 6, a fluorescent lamp is produced and the work function of its cathode is evaluated from the phenomenon of thermionic emission (Patent Document 6).
Herein, the work function serves as a criterion for judging whether or not it is possible to obtain facility of thermionic emission, i.e. excellent properties for a cathode (also called a negative electrode).
The thermionic emission current density J (A/cm2) of a metal is derived from the following equation (Richardson-Dushman equation).J=AT2exp(−eφ/kT)
where A=4πmk2e/h3=1.20×102 (A/cm2K2): Richardson constant, e=1.60×10−19 (J), k=1.38×10−23 (J/K): Boltzmann constant, and φ(eV): work function. T is the absolute temperature of a thermionic emission substance.
According to the Richardson-Dushman equation, for example, the thermionic emission current density of pure tungsten is 4.52×10−5 A/cm2 at 1773K, which is a practically unmeasurable level, while, it is 0.052 A/cm2 at 2273K, 0.15 A/cm2 at 2373K, and 0.40 A/cm2 at 2473K and thus the thermionic emission current does not reach a measurable level unless the temperature is raised.
Accordingly, in order to measure the thermionic emission current of pure tungsten, a cathode temperature of about 2200K or higher is required in terms of normal current measurement accuracy.
As a means for obtaining a high temperature so as to obtain a measurable thermionic emission current, there is, for example, a method of carrying out electric heating using a fine line (Non-Patent Document 2).
Further, other than the measuring methods described above, Non-Patent Document 1 discloses a work function measurement technique using field emission.